Gas turbine driven propeller systems for aircraft propulsion (i.e., turboprops) have been generally known in the industry for a long time but recently there has been a renewed interest in such configurations because of several heretofore unappreciated advantages over conventional jet engines.
Turboprops may not fly as fast as jets but can be much more fuel efficient, especially on smaller planes. In addition, turboprops are generally more effective on shorter flights since they typically have higher rates of climb. One problem with conventional turboprops has been their high noise level but recent development of "pusher" configurations, in which the propellers are placed behind the engine and cabins, have essentially eliminated the problem and produced an exceptionally quiet cabin. However, there are several new problems created by utilizing such a pusher configuration.
One of the major difficulties in designing a pusher propeller installation for modern aircraft, when a gas turbine engine is used as the power source, is to prevent mutual interference between the pusher propellers and the exhaust jet without seriously reducing either the propeller efficiency or jet thrust. Propeller efficiency is typically reduced by about 3 to 7 percent when hot exhaust gasses are dumped into the atmosphere upstream of the propeller blades into their flow path. Further, exhaust of the hot jet through the propeller blades usually introduces vibrational and thermal problems in the blades and simultaneously interferes with the exhaust jet. Modern non-metallic, or even aluminum, propeller blades cannot long resist the hot jet. Even when the exhaust is mixed with cold ambient air to reduce its temperature to only a few hundred degrees, thermal fatigue shortens the life of the blades and, of course, reduces the amount of jet thrust available for propulsion.
Several approaches have been proposed to avoid or solve some of these problems. Deflection of the jet gasses laterally to a point beyond the propeller radius has been tried (see, for example, U.S. Pat. No. 2,604,276) but results in an excessive sacrifice of space in order to accommodate a gas duct of sufficient length and volume to carry the exhaust to a safe distance outboard of the propellers. In addition, the introduction of pronounced bends in the exhaust path, or the placement of the exhaust vent at an angle to the line of flight, leads to losses in jet thrust and to other detrimental effects, such as increased back pressure on the turbine, which reduces the power available for propeller thrust, and decreased aerodynamic efficiency due to higher drag losses.
Another problem with turboprop engines involves proper cooling of the engine nacelle and internal components. Since the propeller system by itself cannot usually supply an adequate flow of cooling air, especially at low air speeds on the ground during idle and when the propellers are "feathered", additional internal cooling fans are usually required. However, the additional weight and power consumption of such fans are detrimental to overall efficiency.
In view of the foregoing, it should be apparent that there is a need in the art for improvements in the design of pusher turboprop engines. Therefore, it is a primary object of the present invention to provide an improved method of, and structure for, ducting turbine exhaust directly out the rear of the engine nacelle without passing through the nacelle sidewalls.
It is also an object of the present invention to provide a turboprop engine in which standard pusher propellers can be separated from the detrimental effects of the hot turbine exhaust gasses.
It is further an object of this invention to achieve such a separation with a minimum of loss in jet thrust or loss in propulsive force from the propellers.
Another object of this invention is to provide an improved method of cooling a turboprop engine at low air speeds by pulling air through the engine nacelle.